Show rich GitHub issue previews with title and rendered body in hovers

GitHub issue references currently use a compact plain-text hover such as `microsoft/vscode#329464 - Open`. This requires opening the issue to understand what it is about, even when the title and body could provide enough context inline.

When hovering a GitHub issue reference or issue pill, show a rich preview containing:

- The full canonical issue title
- The issue body rendered as Markdown, including links, lists, and code blocks
- Basic metadata such as repository, issue number, and open/closed state
- An action to open the issue on GitHub

For long bodies, the preview can use a constrained, scrollable area rather than expanding without limit. While data is loading or unavailable, the current compact hover remains a suitable fallback.

This would make issue references much easier to understand in Markdown documents and agent-generated content without repeatedly switching to the browser.

Related: #335383 requests that Agent Window issue pills preserve their recorded title. This feature request goes further by proposing a rich hover based on live issue data, including the rendered issue body.